(i) Technical Field
The present invention relates to an image forming apparatus to carry out image formation by electrophotographic process including charging, exposing, developing and transferring, and also relates to a process cartridge.
(ii) Related Art
An image forming apparatus of an electrophotographic system generally has constitution and processes as shown below. In the first place, the surface of an electrophotographic photoreceptor (hereinafter sometimes referred to as merely “a photoreceptor”) is uniformly charged to a polarity and potential by a charging unit, and then charges on the surface of the photoreceptor after charging are selectively removed by image exposure to thereby form an electrostatic latent image. Subsequently, toner is adhered to the electrostatic latent image to thereby develop the latent image as a toner image by a developing unit, and the toner image is transferred to a medium to be transferred by a transfer unit, and an image formed is discharged.
From advantages that high speed and high quality of printing can be obtained, electrophotographic photoreceptors are widely used in the fields of duplicators and laser beam printers in recent years. As photoreceptors used in these image forming apparatus, organic photoreceptors using organic photoconductive materials inexpensive and having excellent advantages in productivity and discarding are accounting for main streams as compared with photoreceptors using existing inorganic photoconductive materials such as selenium, selenium-tellurium alloy, selenium-arsenic alloy, cadmium sulfide, etc.
As a charging unit of photoreceptors, a corona charging system using corona dischargers has been used. However, in recent years, a contact charging system having advantages such as low ozone and low power has been put to practical use and widely used.
The contact charging system is a system to charge the surface of a photoreceptor by contacting or extremely approaching a conductive charging member as a member for charging to the surface of the photoreceptor, and applying voltage to the charging member. As a method of applying voltage to the charging member, there are a direct current system of applying direct voltage alone, and an alternating current superimposing system of applying alternating voltage to direct voltage by superimposition. However, this contact charging system has advantages that the apparatus can be miniaturized and harmful gas, e.g., ozone, is hardly generated on one hand, deterioration and abrasion of a photoreceptor are liable to occur by direct electric discharge on the surface of the photoreceptor. Further, in the contact charging system, various foreign matters in the image forming apparatus (e.g., metal powders and carrier lumps) are liable to pierce through the photoreceptor or damage the photoreceptor. As a result, when the photoreceptor is repeatedly used for a long period of time, a high electric field is locally applied to the defect part of the photoreceptor as above at contact charging time and electrical pinhole (pinhole leak) is caused, so that a generation of image defect is liable to occur. Further, as a result of the increment of abrasion of the photoreceptor by contact charging, pinhole leak is liable to be accelerated.
Further, in recent years, for obtaining an image of high image quality, the so-called polymerization toners inclining toward spherical as compared with the shapes of pulverized toners have been often used, but as toners approach a spherical shape, the toners are liable to pass through blade cleaner in the removal of toner, so that it is necessary to closely press the blade cleaner against photoreceptor, which is also the cause of acceleration of abrasion of photoreceptors.
As transfer systems, a system of transferring a toner image directly on paper has been a main stream, but since the degree of freedom of the media to be transferred widens, a system of performing transfer with an intermediate transfer medium is extensively used. However, when an intermediate transfer medium is used, similarly to the above case of using a contact charging system, damaging the photoreceptor is liable to occur. For example, various foreign matters present in the image forming apparatus (e.g., metal powders and carrier lumps) get in between the intermediate transfer medium and the photoreceptor or pierce through the photoreceptor. As a result, when the photoreceptor is repeatedly used for a long period of time, pinhole leak as above is caused, so that a generation of image defect is liable to arise.
Concerning the above issues, it is proposed to provide a protective layer on the surface of an electrophotographic photoreceptor to heighten the mechanical strength.
Since an electrophotographic photoreceptor provided with a crosslinked resin layer, as a protective layer, having an charge transporting property has high strength and a rectifying property, blurring of image is restrained and stable images can be obtained for a long period of time, on the other hand a charge transporting property is controlled by the polar groups at crosslinking terminals, so that residual potential is liable to occur, and the thickness of the protective layer of about 2 to 3 μm is generally used. However, with the thickness of from 2 to 3 μm, duration of life can be lengthened as compared with existing electrophotographic photoreceptors not having a protective layer, but it is not said to be sufficient, and thickening of the protective layer is desired for further lengthening duration of life.
On the other hand, thickening of the protective layer results in the increase of residual potential in the photoreceptor. Since charge is accumulated in the photoreceptor and the accumulated quantity is different between the image exposed area and the unexposed area, the residual potential causes unevenness in electrostatic charge between the image exposed area and the unexposed area at the time of charging in the next cycle, as a result the so-called image ghost, that is, a phenomenon that the previous image pattern remains in the next image pattern, is liable to occur. This phenomenon is liable to occur as the thickness of the surface layer increases, in particular very liable to occur when the thickness is 2 μm or more. Further, in the case of color process using a plurality of toners different in colors, electric field of transfer differs according to the thickness of a toner layer, what is called transfer ghost due to image pattern in transfer is liable to occur, and this is an especially serious issue in obtaining high quality color images.
Further, in recent years, the so-called polymerization toners having a uniform particle size have been used for achieving higher image quality. However, the shapes of polymerization toners are approaching spherical as compared with pulverized toners, and in many cases it is necessary to heighten the electric field of transfer as compared with pulverized toners, so that transfer ghost is liable to occur still more. Further, toners nearer to spherical are small in rolling resistance and easily pass through cleaning members such as cleaning blades, so that cleaning failure is liable to be generated. In many cases the pressure of pressing of cleaning blades is set high as compared with the case of pulverized toners for preventing passing through, but as described above, when the blades are closely pressed against the electrophotographic photoreceptor, the friction with the photoreceptor increases, which accelerates abrasion of the photoreceptor and duration of life is liable to shorten.